This application claims the priority of German Patent Application No. 19645387.9-45, filed on Nov. 4, 1996, the disclosure of which is expressly incorporated by reference herein.
The invention is based on a weld joint of balancing weights on rotationally symmetrical, rotating hollow bodies, as known, for example, from German Patent Document DE 39 31 055 A1.
Normally, rotationally symmetrical, rotating hollow bodies of the type addressed here, particularly hollow shafts or housings of hydrodynamic torque converters, have a certain unbalanced mass as a result of their manufacturing which must be compensated by a locally targeted fastening of a balancing mass selected according to the weight, mostly in the form of a sheet metal strip. The workpieces to be balanced usually consist of weldable steel so that the balancing plates can also be welded on. The fatigue strength of the hollow bodies and/or their wall thickness, as a rule, are also sufficiently high so that welds for the fastening of balancing plates can be easily carried out.
Normally, at least in the series production, the balancing masses are welded on by electric resistance welding, because these welds can be carried out fast, with low and constructionally small tool expenditures and securely with respect to the process. However, this type of welding requires a high denting stability of the shafts at the fastening point of the balancing plate so that the welding electrode can press the balancing plate to be welded on with the required high force against the shaft without any yield of the wall having a softened surface. On the other hand, when the wall thickness is sufficiently large, the zone of the thermal influence which occurs during the welding is small in comparison to the unaffected mass surrounding the zone of the thermal influence and can therefore be tolerated.
In the case of shaft and balancing plate materials, particularly aluminum, which cannot be welded by electric resistance welding, German Patent Document DE 30 35 437 C2 suggests a TIG weld on a small hole in the balancing plate. Here, the zone of the thermal influence is relatively large, and this type of balancing plate fastening is possible only in the case of thick-walled hollow bodies. British Patent Document GB 2 147 388 A recommends to rivet balancing plates to aluminum shafts, in which case a rivet body is previously butt-welded to the respective balancing point; the perforated balancing plate is fitted onto it; and the protruding rivet body end is made into a rivet head. Although this type of fastening permits an unlike pairing of materials, specifically heavy balancing plates made of steel or copper and an aluminum shaft, the welding-on of the rivet body causes a considerable zone of thermal influence. Furthermore, the cold forming-on of the rivet head requires a sufficient denting strength of the hollow shaft. For these reasons, this type of fastening technique is also limited to thicker-walled hollow shafts.
Although it is conceivable to weld balancing plates to the hollow shaft by means of laser welding which is said to leave only a very small zone of thermal influence, during the welding of balancing plates to hollow shafts by means of laser welds, the laser beam would, on the one hand, leave a still relatively large zone of thermal influence which represents a metallurgical weakening which, at least in the case of thin-walled hollow shafts, has an effect which is not negligible, irrespective of whether the weld beam direction is tangential to the balancing plate or to the wall of the hollow shaft or radial thereto. On the other hand, with respect to the investment volume, an expensive laser welding system would not be viable for such small welds. The system would only be used for its purpose, specifically for welding for a very short portion of the overall time. The remaining time, the system would be unused.
Because of the unavoidable zones of thermal influence and/or the force effects on the hollow shafts during the welding, it is therefore very difficult to rapidly and by means of a reliable process balance light-construction hollow shafts. Thin-walled shafts made of steel do not have the required denting strength to be able to withstand the radial contact pressure force of the electrodes during electric resistance welding. Thin-walled steel or aluminum shafts do not tolerate the thermal influence zones occurring during the welding which, in the case of thin-walled components, are very large relative to the direct environment and result in intolerable component weaknesses. Stress tests have shown that balanced, thin-walled shafts regularly fail at the balancing welds. The balancing welds represent metallurgical weaknesses from which the cracks or breaks originate in the case of a failure. According to the applicant's experiences, the problems during the welding-on of balancing plates start no later than at 2 mm when reducing the wall thickness of a corrugated pipe. The applicant has carried out electric resistance spot welding using rectified medium-frequency current and capacitor discharge welds. Despite certain differences in the welding result, metallurgical and partly also mechanical component weaknesses remained in both cases at the weld point which in dynamic stress tests were the starting point of a component failure.
Although in literature soldering or gluing are also mentioned as fastening techniques for balancing plates (compare, for example, German Patent Document DE-PS 725 619), these techniques have not been accepted at least in series production because they cannot be carried out rapidly and in this application are not secure with respect to the process and therefore not durable in every case because of a wide spread of relevant influential factors, such as the physical or chemical surface quality, the joining quality, and the like. Gluing of balancing weights is therefore at most limited to non-weldable materials or other recess applications. The balancing of filament-reinforced hollow plastic shafts also presents problems because at most glued connections are possible here which, however, cannot be carried out rapidly and at least in the case of balancing plate fastenings on the exterior side of shafts vary considerably in their quality.
It is an object of the invention to improve the weld joint of balancing weights of the above-mentioned type such that also thin-walled, rotating hollow bodies can be balanced securely with respect to the process, as well as rapidly and durably in the case of different pairings of the materials of the hollow body and the balancing plate.
Based on the above-mentioned weld joint, according to the invention, this object is achieved by using ultrasonic welding to form the weld joint. Because of the application of ultrasonic welding, a special case of friction welding, the welding heat is generated directly at the contact point itself between the balancing plate and the exterior surface of the corrugated pipe and does not have to be guided from the outside through the cross-sections to be welded together to the contact surface. As the result and because of the very rapid generating of heat by means of the high friction frequency, the zone of thermal influence and the resulting metallurgical stress concentration can be minimized. By means of ultrasonic welds, not only the same types of materials (St/St or Al/Al) can be welded to one another but also very different materials, for example, aluminum to steel or to copper; copper to steel; plastic-coated sheet metal to plastic; and other unlike pairings.
However, in the case of unlike pairings of metallic materials, particularly in the case of specifically heavy balancing weights made of steel or copper on light metal shafts, it must be taken into account that a contact corrosion may occur in the wet area, in which case the light metal, which is lower in the contact series, will be sacrificed. The reason is that, in the case of most current construction metals, the specific weight is connected with the position of the metal in the electrochemical contact series; that is, the heavier a metal, the nobler it is in the sense of this contact series. In order to prevent in such a case--for example, a steel balancing plate on an aluminum shaft--a pitting at the shaft, it is expedient to provide on the contact side of the balancing plate, an electrically insulating insulation layer around the weld and/or to provide a protective varnish on the exposed exterior side of the balancing weight and on the surrounding surface of the hollow shaft which prevent the entering of water into the weld and into the contact zone between the shaft surface and the balancing weight. Especially in the case of thin-walled hollow bodies, a corrosion and also a contact corrosion must be effectively prevented because thin-walled materials, because of their small wall thickness, contain only little "reserve". In rain and mainly in the winter because of the effect of thawing salt, drive shafts used in motor vehicles are very endangered by corrosion and possibly also by contact corrosion.
Although it is known that metals, specifically also steel types can be welded ultrasonically (compare German Patent Document DE 44 31 130 A1 or German Patent Document DE 44 34 132 A1), a person skilled in the art could find no information in this state of the art that the ultrasonic welding of balancing plates is particularly advantageous.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.